fix: disable provenance in Docker build to prevent manifest list creation

The docker-create-manifest job was failing because the AMD64 build was
creating a manifest list instead of a simple image manifest due to Docker
Buildx's default provenance generation. Adding provenance: false ensures
single-platform images are created as simple manifests.

Author: code-inflation

.github/workflows/release.yaml

@@ -62,6 +62,7 @@ jobs:
         with:
           platforms: linux/amd64
           push: true
+          provenance: false
           tags: |
             cybuerg/cfspeedtest:${{ github.ref_name }}-amd64
             cybuerg/cfspeedtest:latest-amd64